MAVEN Accelerometer Observations of Thermospheric Densities During Aerobraking and Deep Dip 2: Wave Features and Connections to Upward Propagating Thermal Tides

Abstract

In early 2019, the Mars Atmosphere and Volatile Evolution (MAVEN) mission underwent an ∼2-month aerobraking campaign during which time the spacecraft periapsis altitude was lowered from its nominal altitude range of 140–160 km to as low as ∼123 km. Excluding spacecraft walk-in/out maneuvers, accelerometer measurements were made along 272 orbits with coverage spanning Ls 340–3°, latitudes ∼5°–54°S, longitudes 0–360°, and local solar time ∼22–17 hr. In this study, we perform a diagnostic analysis of the full aerobraking data set by fitting 4-harmonic waves to mass densities. We then study the variations of these waves as a function of latitude with an emphasis on those observed previously in Mars’ thermosphere by MAVEN and other missions. Additionally, we utilize data collected during the same time period from the Mars Reconnaissance Orbiter’s Mars Climate Sounder to study the vertical propagation of waves originating from the middle atmosphere. Key results indicate that normalized wave amplitudes decrease with latitude, and this is consistent with the latitudinal structure of a diurnal Kelvin mode. We also observe that waves imprinted from the middle atmosphere show normalized amplitude growth with increasing altitude. A complete summary of data sets, analysis methodology, and scientific results is given. The purpose of this study is to add to the body of knowledge surrounding Martian atmospheric wave features and to provide further constraints for future numerical modeling and subsequent tidal mode identification.Plain Language SummaryDue to solar heating, thermal winds flow along the Martian surface where they impact topographic features enabling them to launch waves that propagate upward, often high into the thermosphere (above 100 km). This propagation causes global-scale wave (tidal) motions in local mass densities, and the best way to study how these evolve is through direct observation via spacecraft orbiting the region(s) of interest. The Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft observes these tidal fluctuations in mass densities using its onboard accelerometer. Here, we study the data collected during two unique sampling periods that is, the Deep Dip 2 and 2019 Aerobraking campaigns—where the spacecraft was specifically operated below 150 km. In agreement with previous studies of this kind, our results show that (a) prominent wave components in this lower thermosphere region tend to be consistently organized by latitude, longitude, and altitude zones, and (b) the spatial coherence in wave structures is repeatable across Martian seasons. Key results indicate that normalized wave amplitudes decrease with latitude, and this is consistent with the latitudinal structure of diurnal eastward traveling, vertically propagating tidal components. We also observe that waves imprinted from the middle atmosphere show normalized amplitude growth with increasing altitude.Key PointsAnalysis of Mars Atmosphere and Volatile Evolution accelerometer data collected during aerobraking indicates the presence of longitudinally varying waves in the thermosphereWaves fitted to mass densities show normalized amplitudes decrease with increasing latitudeWaves imprinted from the middle atmosphere show normalized amplitudes grow with increasing altitude